Thermal processing to improve thermal stress resistance

ABSTRACT

Tensile thermal stress failure can be avoided by inducing favorable residual stresses which oppose thermal stresses encountered in the duty cycle. Favorable residual stresses are induced by heating an object to ductile temperature, applying a heat flux to localized areas of the object, and cooling so as to retain thermal stresses.

United States Patent Jack R. Bohn Inventor Palos Verdes Penlnlula,Calll. Appl. No. 20,152 Filed Mar. 16, 1970 Patented Nov. 9, 1971Assignee TRW Inc.

Redondo Beach, Calll.

THERMAL PROCESSING TO IMPROVE THERMAL STRESS RESISTANCE [561 RelerencesCited UNITED STATES PATENTS 1,084.56 l/l9l4 Loss 263/52 PrimaryExaminer-John J. Camby Attorneys-Daniel T. Anderson, Alan D. Akers andJames V.

Tura

ABSTRACT: Tensile thermal stress failure can be avoided by inducingfavorable residual stresses which oppose thermal stresses encountered inthe duty cycle. Favorable residual stresses are induced by heating anobject to ductile temperature, applying a heat flux to localized areasof the object, and cooling so as to retain thennal stresses.

THERMAL PROCESSING TO IMPROVE THERMAL STRESS RESISTANCE SPECIFICATIONThe invention herein described was made in the course of or under acontract with the U.S. Air Force.

In the past thermal treatment has been employed to improve thestructural strength of materials in their duty cycle. The mostfrequently employed thermal processing is stress relief by annealing. Inthe annealing process, stresses built into an article during itsconstruction are relieved by heating after the construction has beencompleted.

in a process analogous to annealing and which improve the structuralstrength of materials, the present process avoids structural failure dueto thermal shock. Tensile thermal shock failure may be avoided byinducing favorable residual stresses which oppose thermal stressesencountered in the duty cycle. Nonsteady state thermal stresses arise asa result of transient temperature gradients in a body and thecorresponding differential thermal expansion which cannot beaccommodated by geometrically compatible displacements within the body.These stresses continuously adjust themselves in such a way that theinternal forces in the body are self equilibrating and the displacementsare compatible. if, in the process, either the stresses or the strainsreach some critical value, failure may occur. Failure can be defined ina variety of ways depending upon the performance requirements of thecomponent. in general, either fracture or excessive deformation may betaken as a critical failure mode.

The term thermal shock has been used by investigators to describecatastrophic brittle fracture which occurs as a result of high tensilestresses which are generated at the cooler side of transiently heatedbodies. The same tensile forces might instead produce excessivedeformation in a body if the material were strong enough to resistfracture, or if the material were ductile rather than brittle. Even ifthe deformation were not excessive during a single heating and coolingcycle, multiple cycling can lead to an accumulated deformation whicheventually will become excessive.

Failure may also involve plastic flow and fracture near the heatedsurface of a body. Post test observations sometimes reveal the presenceof checking or cracking in regions near the heated surface wherecomprehensive plastic flow has occurred during heating. it is herepostulated that the reversal of stress at the hot surface, fromcompressive to tensile, and the reversal of plastic flow fromcompressive to tensile, occurs not only when the body cools but earlierin the cycle, as soon as the temperature gradient begins to disappear.This will happen even if the overall temperature of the body is stillincreasing, as might be the case during sustained heating. Thus thematerial might be put into tension, i.e.. multiaxial tension in mostcases, while it is still very hot, and ductile fracture or hot tearingmight easily take place. Cracking of this nature could also lead to aloss of material at the hot surface which might be mistaken forcompressive spallation or localized erosion in any post test evaluationof a component.

The process according to this invention involves heating a structuralmaterial to the point where the material becomes ductile so that ifforces were applied it would flow plastically. After the material hasreached the temperature of ductility it is subjected to a heat fluxlocally on the surface of the material by any suitable means such aselectron beam induction heating. The temperature gradients employedshould be sufficient so as to induce plastic flow of correct magnitudenear the heated surface of the material. The proper magnitude of theplastic flow is determined by thermal stress or trial and errorexperiments. The cooling period is regulated to reduce relaxation ofresidual stresses. it is desired to retain permanent thermal strain,however forced cooling is not essential to reduce the possibility ofrelaxation and creep.

One of the chief advantages of this process is that it is applicable tomaterials of any configuration. Whereas in the past, articles of certainconfiguration only could be processed prestressed because mechanicalrestraints which conform to the shape of the article were required, thepresent method is free of such restrictions. Articles of anyconfiguration may be heated to an elevated temperature, where it isductile and subjected to a flux to induce permanent thermal strain, andcooled. No mechanical restraint are necessary.

The method which has beendescribed relies on the fact that mostrefractory and ceramic materials which are weak and brittle at roomtemperature exhibit plasticity when heated to elevated temperature. Thusin a specific example, if the nozzle insert of a rocket engine is heatedslowly to a temperature where the material is ductile and then subjectedto a severe shock environment, fracture will not occur, but instance thematerial will undergo plastic deformation involving compression at theinside diameter and tension at the side diameter. Upon cooling toambient temperatures, the inside diameter will be in a state of residualtension and the outside diameter will be in residual compression. Themagnitude of these residual stresses ,will be influenced by thestress-strain behavior of the material in-tension and compression forthe temperature from which the thermal shock processing treatment isinitiated, the severity of the subsequent thermal shock treatment, andthe cooling rate to ambient. In the application duty cycle the resultingstresses will be lower by the magnitude of the residual stresses whichwere inducted. Care must be taken during the thermal processing not toinduce tensile residual stresses which exceed the fracture strength ofthe material.

The following table sets forth specific embodiments which furtherillustrate the invention.

S. Specimen which survived in 3.

b. No prior thermal shock treatment (6.000 B.t.u.lft.' sec.) Pulsed fromrm temp to power (4,800 Btu/ft. sec.) Pulled from rm temp to full power(6.000 B.t.u.lft. sec.)

survived without cracking Failed cstsstrophicslly at 0.04 see.

From the above table it can be seen that samples which did not receivethe heat flux pulse starting from isothermal conditions at some elevatedtemperatures could not withstand thermal shock as severe as those whichwere thermally processed.

I claim:

1. A process for the improvement of thermal shock resistance of anarticle comprising:

A. heating an article to a temperature where it is ductile B. applying aheat flux to a localized area on thesurface of said article to inducepermanent thermal strains throughout the body, and C. cooling saidarticle to retain then'nal stresses.

2. A process for the improvement of thermal shock resistance of anarticle comprising:

A. heating an article to its ductile temperature,

B. applying a heat flux to localized areas on the surface of I saidarticle so as to induce a predetermined plastic flow near said surface,and

C. cooling said article at a predetermined schedule effective to retainthermal stresses.

3. A process according to claim 2 wherein the heat flux applied is inthe magnitude of the heat which the article will experience in theapplication duty cycle. 5

1. A process for the improvement of thermal shock resistance of anarticle comprising: A. heating an article to a temperature where it isductile B. applying a heat flux to a localized area on the surface ofsaid article to induce permanent thermal strains throughout the body,and C. cooling said article to retain thermal stresses.
 2. A process forthe improvement of thermal shock resistance of an article comprising: A.heating an article to its ductile temperature, B. applying a heat fluxto localized areas on the surface of said article so as to induce apredetermined plastic flow near said surface, and C. cooling saidarticle at a predetermined schedule effective to retain thermalstresses.
 3. A process according to claim 2 wherein the heat fluxapplied is in the magnitude of the heat which the article willexperience in the application duty cycle.